The present invention relates to a material in the form of a sheet, a foil or a strip of aluminum or an alloy thereof, which has first been mechanically and then electrochemically roughened on one or both surfaces. The invention also pertains to a process for the production of this material and to its use as a support material in the manufacture of offset-printing plates.
Support materials for offset-printing plates are coated, on one or both sides, with a radiation-sensitive layer (reproduction coating). The coating is applied either directly by the user or by the manufacturer of precoated printing plates. This coating permits the production of a printing image of an original by a photo-mechanical route. Following the production of this printing form from the printing plate, the coating support comprises image areas which are ink-receptive in the subsequent printing process. Also, simultaneously with the image-production, a hydrophilic image-background for the lithographic printing operation is formed in the areas which are free from an image (non-image areas).
Thus, a coating support for reproduction coatings used in the manufacture of offset-printing plates must meet the following requirements:
Those portions of the photosensitive coating which have become comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue and without any stronger attack on the support material by the developer.
The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The photosensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure.
Suitable base materials for coating supports of this kind fundamentally include aluminum, steel, copper, brass or zinc foils, and also plastic sheets or paper. By appropriate modifications, such as, for example, graining, matte chromium-plating, surface oxidation and/or application of an intermediate layer, these base materials are converted into coating supports for offset-printing plates. The surface of aluminum, which is presently the most frequently used base material for offset-printing plates, is roughened according to known methods, e.g., dry-brushing, slurry-brushing, sandblasting, or chemical and/or electrochemical treatment. In order to increase the resistance to abrasion, the roughened substrate may additionally be treated in an anodizing step to produce a thin oxide layer.
A printing-plate support provided with a radiation-sensitive coating must, moreover, meet additional requirements, some of which are correlated to the requirements demanded of the support material itself. These requirements include, for example, high radiation-sensitivity (photosensitivity), good developability, clear contrasts after exposure and/or developing, long print runs and a reproduction which is as far as possible true to the original. Particularly in printing plates carrying positive-working radiation-sensitive coatings, a substantially halation-free behavior of the radiation-sensitive coating during irradiation (exposure) of the printing plate and a good water/ink balance of the printing forms (i.e., smallest possible amount of water used and highest possible variation tolerance in water requirement during printing) also play an increasingly important role. From the state of the art, the following publications, for example, are known which provide solutions of a few of the problems, such solutions including, on the one hand, production of elevations in or on the radiation-sensitive coating and, on the other hand, a combination of several roughening steps for the support material:
German Offenlegungsschrift No. 2,512,043 (equivalent to U.S. Pat. No. 4,168,979) discloses a radiation-sensitive printing plate, in which the surface of the radiation-sensitive coating is provided with a matte layer, which is removed upon developing. This matte layer is, generally, a binder layer (e.g., composed of a cellulose ether) which has matting particles dispersed therein, comprising, for example, SiO.sub.2, ZnO, TiO.sub.2, ZrO.sub.2, glass, Al.sub.2 O.sub.3, starch or polymers. A printing plate constructed in this way is supposed to reduce the time which is required for obtaining a substantially complete and uniform contact between the film original and the radiation-sensitive coating, in the exposure step of the process for manufacturing printing forms.
German Offenlegungsschrift No. 2,926,236 (equivalent to South African Pat. No. 80/3523) discloses a radiation-sensitive copying material comprising, in its positive-working radiation-sensitive coating, particles the smallest dimension of which is at least equal to the thickness of the layer itself and which correspond in quality to the particles described in the above-mentioned German Offenlegungsschrift No. 2,512,043. Such a material should be suitable for any application in which positive contact copies must be produced in a vacuum frame in and which high image resolution and true reproduction of the original are important. In particular, it is stated that the material shows a reduced tendency to halations in the copying process, i.e., halations (lateral and oblique incidence of radiation) may occur during irradiation, as a result of a locally increased distance between the original and the radiation-sensitive coating, and will lead to an imprecise reproduction of small image elements, for example, halftone dots.
However, the process step of applying particles together with a binder to the radiation-sensitive coating or of incorporating particles into the coating without any special binder is expensive and requires a great deal of accuracy, in particular in modern continuously working coating devices. During developing of the coating, the particles which have been applied or admixed to the coating, moreover, present a sort of "foreign matter" to the developing liquid and particularly also to the automatically operating processors and may interfere with the operational flow. The additives, furthermore, do not have any particular effect on the water/ink balance of the printing form.
In the process for the continuous production of a lithographic surface on a metal strip by wet grinding and electrochemical treatment in an electrolyte, according to German Pat. No. 1,962,728 (equivalent to U.S. Pat. No. 3,691,030), the electrolyte is used for wetting during grinding and electrochemical treatment is carried out after grinding. In the process, both grinding and electrochemical treatment can, in each case, produce a roughening effect, for example, on aluminum.
The process for the manufacture of a support for lithographic printing plates, according to German Offenlegungsschrift No. 3,012,135 (equivalent to UK Patent Application No. 2,047,274) is carried out in at least three steps, comprising (a) mechanically roughening the aluminum sheet, (b) removing from 5 to 20 g/m.sup.2 of aluminum from the roughened surface and (c) performing an electrochemical roughening treatment, in which electric current of an alternating wave-form is applied in an acidic aqueous solution and in which this current must have specific parameters. Electrochemical roughening may be followed by another abrasive treatment and also by an anodic oxidation of the roughened surface. The surface topography of the support must be such that the primary structure of the surface shows uniform mounds, onto which a secondary structure of pinholes is superimposed. The bisecting axis of each pinhole is approximately perpendicular to the tangent line at the outer face of the corresponding mound. The approximate statistical distribution of pinhole diameters is such that 5% of the holes have a diameter D.sub.5 of not more than 3 microns and 95% of the holes have a diameter D.sub.95 of not more than 7 microns, i.e., the bulk of the holes have diameters in the range between 3 and 7 microns, particularly between 5 and 7 microns. The density of pinholes is approximately 10.sup.6 to 10.sup.8 holes per square centimeter. In addition to roughening with a rotating nylon brush with the application of an aqueous pumice slurry, which is the preferred method of roughening and is employed in the only example, it is stated that wire-brushing or ball-graining of the surface may also be used in the mechanical-roughening step; however, this statement is not further specified. Before the abrasive treatment step, the mechanically roughened aluminum has a center line average roughness R.sub.a of 0.4 to 1.0 micron.
Japanese Published Patent Application No. 123 204/78 (Application No. 38238/77, published Oct. 27, 1978) also described a combination of mechanical roughening by nylon brushes with an aqueous pumice slurry and electrochemical roughening, which is used for aluminum support materials for printing plates. An abrasive treatment is carried out after the completion of the two roughening steps, but not between these steps.
British Pat. No. 1,582,620 discloses a combination of (a) mechanically roughening and (b) electrochemically roughening support materials for printing plates by means of an alternating current in an aqueous solution containing HCl and/or HNO.sub.3. A detailed specification of the topography of the surface, in respect of quality or quantity, is not given. In the examples, mechanical roughening of aluminum exclusively comprises roughening with oscillating nylon brushes and with the application of an aqueous slurry containing pumice and quartz; the specification also mentions wire-brushing as an alternative method, which is, however, not explained in detail. The aluminum surface is chemically cleaned between the mechanical and electrochemical roughening steps.
The support material for printing plates comprising aluminum, according to U.S. Pat. No. 2,344,510, is initially mechanically roughened, in particular by wire-brushing, and is then chemically or electrochemically roughened. In the procedure, the finer lithographic grain resulting from chemical or electrochemical roughening is to superimpose itself upon the relatively coarse lithographic grain resulting from mechanical roughening. Between the mechanical and the preferred electrochemical roughening treatment, a cleaning step is carried out, which uses a 5% strength aqueous NaOH solution, at 95.degree. C. The roughening electrolyte comprises an aqueous solution containing NaCl and HCl. Roughening may be followed by an anodic oxidation of the material.
U.S. Pat. No. 3,929,591 describes a support material for printing plates which comprises aluminum and is produced in three steps, i.e., (a) a mechanical roughening treatment with the application of a wet mass of abrasive particles based on silicates, oxides or sulfaces, (b) an electrochemical roughening treatment using an alternating current in an aqueous electrolyte containing phosphates or H.sub.3 PO.sub.4 and (c) an anodic oxidation treatment using direct current in an aqueous electrolyte containing H.sub.2 SO.sub.4. Step (b) is intended to produce an increase in reflectance of the surface by at least 5%. A detailed qualitative and quantitative specification of the surface topography is not given.
A combination of mechanical and electrochemical roughening may result in an improvement of the water/ink balance; however, the prior art nowhere mentions or suggests that this combination has any effect on a substantially halation-free behavior of radiation-sensitive printing plates so produced. In addition, the comparative tests described below show that an aluminum support for printing plates, even though it has been mechanically roughened or even wire-brushed and then electrochemically roughened and, optionally, subjected to an anodic oxidation treatment of the surface, will by no means yield, on the one hand, a good water/ink balance during printing with these printing forms and, on the other hand, an at least reduced tendency to halation in the manufacture of the printing forms.